Gas plasmas are widely used in a variety of integrated circuit (IC) fabrication processes, including plasma etching and plasma deposition applications applied to a semiconductor substrate. Generally, such plasmas are produced within a processing chamber by introducing a low-pressure process gas into the processing chamber and then directing electrical energy into the chamber for creating an electrical field therein. The electrical field creates an electron flow within the chamber which ionizes individual process gas molecules by transferring kinetic energy to the gas molecules through electron-molecule collisions. The electrons of the electron flow are accelerated within the electric field in the processing chamber for efficient ionization of the gas molecules, and the ionized molecules of the process gas and free electrons collectively form what is referred to as a gas plasma or plasma discharge.
The gas plasma created within the processing chamber may be utilized without any additional process gases, such as for etching the exposed surface of a substrate, or it may be used in combination with other selected process gases for depositing various material layers onto a substrate. For example, within an etching process, the ionized plasma particles will generally be positively charged, and the substrate will be negatively biased such that the positive plasma particles are attracted to the substrate surface to bombard the surface and thus etch and remove a layer of material therefrom.
If it is necessary to deposit thin material films or layers onto the substrata, such as to provide conductive and ohmic contacts for IC fabrication, a deposition process, such as chemical vapor deposition (CVD) may be used. In CVD, process gases are pumped into the processing chamber and the gases chemically react proximate the substrate to form reaction by-products which then deposit on the substrate to form the desired material layer. A CVD process using a gas plasma is generally referred to as a plasma-enhanced CVD or PECVD process. PECVD is often used, for example, for lowering the process temperatures and thermal energy that are usually necessary for a proper chemical reaction with standard CVD. In PECVD, electrical energy delivered to form and sustain the plasma reduces the thermal energy necessary for the chemical reaction.
One common hardware configuration for plasma etching and PECVD is referred to as a parallel plate RF discharge device. In such a device, a planar substrate support and a planar gas supply element, such as a showerhead, are oriented generally parallel with each other in a processing chamber. One or both of the electrodes are electrically biased with RF energy to operate as opposing RF electrodes for energizing one or more of the process gases into an ionized plasma. The distance between the electrodes is relatively small with respect to the dimensions of the electrodes, and the distance may be around 1 inch, for example. The process gas is introduced through small holes within the showerhead electrode, and RF power is applied to the showerhead, requiring that the showerhead be insulated from any ground reference. One such PECVD process and showerhead structure is disclosed in U.S. Pat. No. 5,567,243, which is commonly owned with the present application. Another suitable showerhead structure is disclosed in U.S. Ser. No. 08/940,779, entitled "Apparatus and Method for Preventing the Premature Mixture of Reactant Gases in CVD and PECVD Reactions," which is also commonly owned with the present application. Both the issued patent and pending application are completely incorporated herein by reference in their entireties.
The ground reference for the parallel plate device is generally the metal processing chamber in which the electrodes are disposed. The substrate support electrode may or may not be grounded as well. An insulator, in the form of a plate of insulative material (e.g. quartz) is positioned between the grounded chamber and the showerhead electrode. The electrodes and insulator plate are usually flat, planar structures, although they may have some curvature to them. Since the process gas is passed to the showerhead electrode, it is necessary for the gas to pass through the insulator plate. However, the holes or openings which must be formed in the insulator plate to allow gas passage to the showerhead electrode, may be detrimental to the stability of the plasma.
More specifically, the openings through the insulator provide a plasma breakdown path between the biased RF electrode and the grounded processing chamber. The plasma breakdown occurs when plasma is formed within the openings and creates an electrically conductive path between the RF showerhead electrode and a ground reference, such as the chamber lid or some other portion of the processing chamber. The plasma will then have a tendency to arc to ground, which detrimentally affects the stability of the plasma and, as a result, the stability of the plasma process. The plasma breakdown usually occurs at a particular RF power level and system pressure and thus limits the RF power that may be applied to the plasma discharge. The limitation of plasma power will reduce the density of the plasma. The relationship between the breakdown power and the gas pressure is determined by various system parameters, such as the dimension of the processing chamber, the RF frequency, and the type of process gas utilized.
An additional drawback to the existence of a breakdown plasma in the insulator openings occurs when the processing system is utilized for PECVD. In such a case, a plasma discharge in the openings may lead to deposition of a conductive coating therein. This may further lead to plasma instability and is a problem even under processing conditions where plasma arcing and breakdown does not actually occur.
Some techniques have been employed to prevent a plasma breakdown; however, such techniques generally complicate the fabrication of the plasma processing system, and thus increase the overall expense of the system. For example, the insulator plate may be made thicker to increase the lengths of the openings in the plate. Furthermore, the openings may be profiled with grooves or flutes to lengthen the effective path length through the openings. Still further, the openings might be angled to also lengthen the effective path length through the openings. Such techniques increase the complexity of the insulator plate construction, and therefore, increase the fabrication costs of the plate.
Another solution to the problem of breakdown voltage is to maintain the pressure in the processing chamber within a range that will allow a higher amount of RF power to be delivered to the plasma without breakdown. However, such a limitation also limits the operation of the parallel plate device and its applications in plasma processing.
Accordingly, it is an objective of the present invention to maintain a stable plasma within a parallel plate discharge device over a wide range of process conditions and pressures.
To that end, It is another objective of the invention to reduce and prevent plasma breakdown within a parallel plate device using an insulator between the showerhead electrode and the ground reference.
It is still another objective of the invention to reduce and prevent plasma breakdown within a parallel plate device without increasing the overall cost and complexity of the device.
It is another objective of the invention to reduce plasma deposition of a conductive coating within the opening in an insulator plate of a parallel plate device used in a PECVD process.